Respiratory system compliance (C.sub.rs) is a measure of the elastic properties of the lung and chest wall. One method of measuring C.sub.rs involves training conscious adults to relax their respiratory muscles to eliminate the perturbations they cause. However, it is difficult to achieve consistent results with such techniques due to the level of training required, and such training is impossible in infants. Respiratory system compliance can be measured in anesthetized subjects without concern for respiratory muscle contraction, but anesthesia lowers lung compliance and thus also lowers the overall respiratory system compliance. In the relaxation methods requiring training mentioned above, a subject inspires to a given volume, then relaxes against a closed mouthpiece while the pressure at the mouth and lung volume are recorded.
Another method of measuring respiratory system compliance employs externally applied pressures. A subject is observed while breathing normally and then either airway opening pressure is increased or body surface pressure is decreased, and the subject's breathing is again observed. Respiratory system compliance is calculated from the pressure change data and observed changes in the subject's lung volume. This latter method is described as a dynamic measure of compliance since the patient is breathing, while the above-described methods requiring training the patient to relax provide a static measure of compliance, since expiration results from the compliance of the lungs without the effects of chest muscle interaction.
Another technique for measuring static total respiratory system compliance is known as the pulse method. See Suratt et al., "A pulse method of measuring respiratory compliance," J. Appl. Pysiol. 49(b):1116-21 (1980). In the pulse method, a pulse airflow is introduced at the end of expiration, and the pressure at the mouth is measured during the pulse. By plotting volume vs. pressure, and calculating the slope of the resulting line, respiratory system compliance can be determined. In other words, compliance is calculated by dividing the airflow rate by the change in transrespiratory pressure. However, this method requires a subject be relaxed to some extent; in a unrelaxed subject, the pressure-volume plot will be non-linear. The pulse flow method is also useful for measuring static lung compliance (C.sub.L). See, Suratt et al., "Lung compliance and its transient elevation," J. Appl. Pysiol., 50(b):1318-24 (1981).
Static respiratory system compliance can also be determined while a subject is under sedation. See, Grunstein et al., "Expiratory volume clamping: a new method to assess respiratory mechanics in sedated infants," J. Appl. Pysiol. 62(5):217-14 (1987); U.S. Pat. No. 4,802,492--Grunstein incorporated herein by reference. In this method, the patient breathes through a two way valve such that expiration may be selectively blocked. When expiration is blocked, the inspiration flow path remains open, permitting the volume and airway pressure within the respiratory system to increase. However, in infants in particular, the Hering-Breuer reflex produces apnea at lung volumes above the resting end-expiratory level. The progressive recruitment of the Hering-Breuer reflex enables passive respiratory mechanics to be noninvasively determined over a wide range of lung volume. The expiratory volume clamping technique relies on expiratory occlusion accomplished by means of the two way valve system having separate inspiratory and expiratory ports described above. The expiratory valve is closed during normal breathing of a sedated patient and the volume, airflow and airway pressure are recorded. The occlusion is then suddenly released during an expiratory phase and the same data are recorded after release; these data are indicative of passive exhalation. As discussed by Grunstein et al., "Expiratory volume clamping: a new method to assess respiratory mechanics in sedated infants," J. Appl. Pysiol. 62(5):2107-14 (1987); and U.S. Pat. No. 4,802,492--Grunstein, referenced above, the determination of volume and pressure over an extended lung volume range permits the net respiratory system compliance to be determined. In addition, these same data permit the passive time constant (.tau..sub.rs) to be derived from the slope of the volume vs. flow curve.
Although the methods disclosed by these references provide valuable and repeatable data, many of them provide information from a static state. Ideally, a passive and dynamic system would be provided to measure respiratory system compliance. Static methods produce no flow, while flow would occur in a dynamic system. Dynamic information is important since the effective stiffness of the lungs is influenced by the airways within the lung, e.g., the bronchi, bronchioles, and the resistance of these airways caused by the degree of narrowing therein. For example, when the airways are narrowed due to disease, e.g. asthma, emphysema and chronic bronchitis, the more rapidly breaths are taken the less the lungs expand. Currently, there is no readily applicable test of smaller airway function and their influence on the dynamic compliance of the lungs. Thus, it is an object of the present invention to measure the passive, dynamic mechanical properties of the respiratory system, particularly the respiratory systems of infants.
Moreover, it would be desirable to measure the passive time constant of the respiratory system and thus derive another measure of the resistance of the respiratory system. It is accordingly a further object of the present invention to provide methods and apparatus to collect and process such data.
Finally, in the case of infants, medicines such as bronchodilators used for treating conditions such as asthma are simply nebulized with the air in a breathing mask placed gently over the patient's face. However, droplets of medicine typically condense and collect in the mouth and throat region, never reaching the lungs, and in particular never reaching the smaller passageways within the lungs where the medicine will be the most efficacious. Therefore, it is yet another object of the present invention to provide methods and apparatus for effectively introducing a medicament into the lungs of a patient, particularly an infant.